Problem: Which of these numbers is prime? ${9,\ 27,\ 54,\ 71,\ 99}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 27 are 1, 3, 9, and 27. The factors of 54 are 1, 2, 3, 6, 9, 18, 27, and 54. The factors of 71 are 1 and 71. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 71 is a prime number.